This invention relates to the production of flexible unfired ceramic green sheets which are used for lamination in a multilayer ceramic substrate, and more particularly, to a novel method of forming a ceramic slurry (also known as slip or paint) for casting into the green sheets.
Due to the high packing densities attainable with multilevel ceramic (MLC) substrate structures, they have achieved widespread acceptance in the electronics industry for packaging of semiconductor integrated devices. The general processes of fabricating a MLC substrate are well known in the art and are described in the publication entitled "A Fabrication Technique for Multilayer Ceramic Module", by H. A. Kaiser et al, Solid State Technology, May 1972, pp 35-40; IBM Technical Disclosure Bulletin entitled "Forming Green Sheet Ceramic" by C. M. McIntosh and A. F. Schmeckenbecher, Vol. 19, No. 3, pp 945-946, August 1976, and U.S. Pat. Nos. 3,899,554 and 4,109,377. The basic building block used in the MLC fabrication is the unfired ceramic green sheet, nominally 0.2 mm or 0.28 mm thick. A key factor in achieving acceptable MLC substrate yields is the formulation of a green sheet that exhibits the necessary strength for handling and processing and the necessary density and dimensional stability to ensure accurate plane-to-plane registration when the sheets are stacked and laminated. In addition, the green sheet must have sufficient compressibility and bond strength.
The green sheet properties are predominately dictated by the nature and composition of the slurry from which the green sheet is prepared. Specifically, green sheet density and fired shrinkage are controlled by the amount of deagglomeration of the slurry. To expound on this, the slurry is normally formulated in accordance with the prior art practice, from a ceramic particulate, a binder resin system and a solvent system. The function of the binder resin system is to provide adhesive and cohesive forces to hold the ceramic particulate together in its green sheet configuration. The solvent system is of volatile composition whose role is to dissolve the binder resin system into solution, to aid in uniformly mixing the binder resin with the ceramic particulate and to provide the necessary viscosity to the resultant ceramic paint for subsequent casting. The finely divided, low dielectric ceramic particulate forms the substrate material in the ultimately fired structure.
The ceramic particulate may be selected from a number of materials, depending on the property desired in the fired ceramic end product. Typical ceramic particulates include alumina, steatite, aluminum silicate, zirconium dioxide, titanium dioxide, magnesium silicate, bismuth stannate, barium titanate and their combinations.
The binder resin system will normally be comprised of a basic solvent soluble thermoplastic organic polymer having film forming properties which is nonvolatile at moderate temperatures but which will volatilize with other constituents of the resin system on firing of the green sheet to the final sintered state. Typical binders include polymethyl methacrylate, acrylates, polyvinyl acetate, polyvinyl butyral and nitrocellulose.
The binder resin system may contain other additives such as plasticizers which are soluble in the solvent mixture and which are volatilized during firing of the green sheet into its sintered state. The use of a plasticizer imparts flexibility to the polymer film and, in turn, to the green sheets to maintain it flexible, moldable and workable prior to firing. A wide range of plasticizers may be employed in the binder system and the selection may be made in accordance with techniques well known in the art.
The solvent system or mixture is a volatile fluid whose function is to completely dissolve the binder resin system into a "binder solution" to effect uniform mixing of the binder system with ceramic particulate, and to provide sufficient fluidity to the ceramic slurry for subsequent casting into a cohesive sheet.
The phrases "green sheet density" and "particle packing density" are used synonymously herein and refer to the final density of the cast ceramic green sheet.
The most versatile conventional method of forming a slurry is to thoroughly blend in a ball mill the ceramic particulate, binder resin and solvent system to coat the ceramic particulates with the binder resin and provide a smooth uniformly dispersed slurry. To elaborate on the details of the ball milling technique, attention is focused on a specific combination of basic materials for slurry preparation. The basic materials selected consist of alumina (Al.sub.2 O.sub.3) and glass frit representing the ceramic particulate, methylisobutyl ketone and methyl alcohol representing the solvent system, and polymeric polyvinyl butyral in combination with a plasticizer, e.g. dipropylene glycol dibenzoate, representing the binder system. In the prior art method, all of the methyl isobutyl ketone, methyl alcohol and plasticizer and a small portion of the polyvinyl butyral are initially mixed into a liquid mixture having a suitable viscosity. Then, the liquid mixture and the glass frit, whose particle size is typically about minus 200 mesh or 9-11 m, are milled in a ball mill for a period of 4-8 hours to reduce the size of frit particles to about 3-3.5 m and form a new mixture. Next, the remaining portion of the dry, undissolved polymeric binder material and all of the alumina are added to the new mixture and the milling operation is continued for about 12-22 hours to disperse the alumina in the new mixture and form a slurry having a desired viscosity and a preferred particle packing density which translates into a green sheet density, upon casting of the slurry into green sheets, of about 2 grams/cc.
This conventional method has several shortcomings. First, since a large portion of the polyvinyl butyral is added in a dry, undissolved state to a fairly viscous medium, during the initial mixing, polyvinyl butyral folds over in such a manner that lumps of polyvinyl butyral having a core of dry polyvinyl butyral powder and an outer shell of dissolved or viscous polyvinyl butyral are formed. Due to the rolling, cascading, churning and grinding operation of the ball mill, these lumps of polyvinyl butyral in the presence of the alumina grinding media transform into large, inert armor coated type balls which fall to dissolve in the slurry even after an extended dispersion milling period. The formation of these inert balls deletriously affects the slurry viscosity, alters the milling efficiency of the ball mill and the batch of green sheets that are cast from the slurry will suffer from undesirable green sheet properties which will alter the sintered characteristics of the batch. Likewise, the next batch of green sheets that are cast from the next job of slurry that is prepared in the same ball mill will have undesirable characteristics due to an excess of polyvinyl butyral content which results from the dissolution of the polyvinyl butyral balls from the first batch in the liquid solvents of the second batch. Another shortcoming of the prior art method of slurry preparation is that since alumina is dispersed in a rather highly viscous mixture, minor variations in alumina deagglomerates, particle size and impurities, for e.g. due to lot-to-lot variations, cause significant variation in the milling efficiency and thereby cause a variation amount of deagglomeration that can be achieved which again translates into variation in the green sheet density and shrinkage upon sintering. Another drawback of the prior art process is that it is a rather time consuming process requiring 16-30 hours to prepare the slurry.
It would appear that one way of reducing or eliminating the above polyvinyl butyral ball formation problem is to physically separate the polyvinyl butyral particles and prevent their lumping together on initial contact with the solvent mixture by dispersing the polyvinyl butyral particles in the alumina i.e. dry blend the of polyvinyl butyral powder and all of the alumina particulate together and adding this combination to the ball mill containing the mixture of solvents, glass frit, etc. However, this procedure does not alter the effect of time dependent solubilization of the polyvinyl butyral which affects the rate of dispersion.
Accordingly, it is an object of this invention to provide a process of forming a slurry for casting into green sheets which is free of the above prior art process sensitivites.
Specifically, it is an object of this invention to provide a method of forming a green sheet casting slurry having a high degree of deagglomeration.
Another object of this invention is to provide an efficient milling process of forming a slurry for casting into ceramic green sheets.
It is a yet another object of this invention to provide a method of forming a ceramic slurry having stability and consistency not only within a given batch, but also from batch to batch.